The present invention relates to the real time measurement of a fluid volume in a blood filter, and more particularly to measurements of blood volume and changes in blood volume in a filter.
In a large number of medical procedures, at least a portion of the blood volume is passed through a filter. The filter is designed to remove certain particulate matter from the blood. Alternatively, the filter may be designed to remove specific chemicals and water solutions during the filtering process.
A typical device that exposes blood to a filter is a dialyzer. One of the main parameters of filter quality is filter blood volume (FBV). In terms of the dialyzer, the filter blood volume is often referred to as dialyzer blood volume (DBV). Alternatively, the volume has been referred to as fiber bundle volume. The term filter blood volume (FBV) will be used to encompass all the filter constructions including filters having plates, fibers or biological cells.
Due to the exposure to the blood, clotting may occur in the filter and thereby significantly decrease the surface area of exchange available to the blood and hence decrease filtration. In this case the quality of treatment may be jeopardized. Fiber clotting during hemodialysis can significantly decrease the volume of blood for solute exchange.
Filter bundle volume, the total space within the blood compartment of hollow fiber hemodialyzers, correlates closely with dialyzer surface area, a major determinant of solute clearance. A decrease in surface area due to clotting causes a decrease in solute clearance that puts the patient at risk for inadequate dialysis. Since FBV correlates with membrane surface area and is easy to measure in vitro, FBV has been selected, in centers where dialyzer reuse is permitted, as the main criterion that allows a dialyzer to be reused. In countries where reuse of dialyzers is permitted, the main criteria for reuse is comparison of current FBV to its initial value. Traditionally, FBV is measured after cleaning, clot removal, and pressure flushing. The FBV values determined by this traditional process may not represent the actual FBV.
The sequence of procedures for dialyzer reuse consists of cleaning, including flushing the fibers with pressurized water to remove clots and debris, and measurement of FBV by volumetric displacement of air or liquid which is compared with the original dialyzer volume. Because vigorous flushing removes much of the clot, FBV measured in vitro may not represent the true in vivo volume and the corresponding surface area available for solute exchange.
As the filters must be monitored and changed, the filters are usually disposed extracorporeal. The relationship of a filter to a particular medical procedure is sensitive to a variety of parameters. Generally, the operating pressure, or resistance of a filter must be within particular predetermined limits.
However, as the filter is often removing material that is not perceptible to the naked eye, it is general practice to remove the filter from the fluid circuit and fill the filter with water to determine the filter volume. This process is not only time consuming, but exposes the filter to contamination. Also, the interruption of the filtering process can be detrimental to the treatment of the patient. For example, in procedures such as hemodialysis, a filter is used to remove selected particles and liquids from the bloodstream of the patient.
Difficulties in making such measurements have resulted from the fact that such procedures usually involve extracorporeal circulation of the blood from a patient through, for example a blood treatment system, and in many cases the effects of the system itself on the blood flow or on the measurement devices is unknown. For example, if blood is directed to a dialysis filter through plastic tubing, the effect of the plastic material on measuring equipment using ultrasound waves may not be known with any certainty, since characteristics of the material can vary from one tube to another.
There is emerging technology of locating a blood filter inside the patient. However, even these filters are subject to the traditional concerns of proper functioning. If the filter is inserted into the body, then the measurements are made using sensors mounted on a blood vessel.
Therefore, the need exists for a method and apparatus for monitoring the FBV. A need also exists for the real time monitoring FBV. A need further exists for measuring the FBV during use or when the filter is not in use, wherein the FBV volume has improved accuracy. The need also exists for monitoring a change in the FBV to allow adjustments to the procedures to accommodate or correct such volume changes.
It is an object of the present invention to provide a method and apparatus for accurately and reliably measuring filter blood volume in a filter. The filter may be a patient filter in the patient, extracorporeal, in a separate machine, dialyzer, or in a testing procedure of the filter in a separate machine, such as a reuse machine. In the separate testing procedures, the FBV be determined with any liquid. Therefore, blood not is required to determine the FBV. The present invention also allows for FBV determination and particularly externally of the vein or artery, or in tubing leading to a blood treatment system which carries the blood exteriorly of the body of the patient.
It is another object of the present invention to measure a filter blood volume by measuring an induced dilution in the blood and monitoring passage of the dilution.
Generally, the present invention relates to a method and apparatus for measuring or monitoring the volume of a blood side of a filter by one of (i) employing a bolus injection in the blood side upstream of the filter; (ii) changing the filtration rate in the filter, or (iii) employing a bolus injection in the dialysate side.
Briefly, in the bolus injection configuration, a bolus is introduced in the blood side upstream of the filter during operation of the filter. A downstream signal corresponding to passage of the bolus downstream of the filter is obtained. Further, a blood flow rate through the filter determined. Finally, the blood volume of the filter is calculated in response to the downstream signal and the determined blood flow rate. In a preferred embodiment, the bolus is introduced in the blood side upstream of the filter sufficiently near the filter to substantially preclude compensation of the downstream signal.
In the filtration rate change embodiment for monitoring of a volume of blood in a dialysis blood filter during operation of the filter, a blood parameter is measured downstream of a blood side of the filter; a blood flow rate through the blood side of the filter is determined; the filtration rate is changed to change the blood parameter; and the blood volume is calculated in response to the change of the blood parameter and the determined blood flow rate.
In the bolus injection in the dialysate side embodiment, the filter blood volume is calculated from the introduction and passage of a diffusable and non diffusable indicator through the blood volume and the dialysate volume.
The present invention includes, in part, injecting a volume of a diffusable indicator into a blood flow, wherein the diluting effect of the indicator over a period of time is accurately determined by a sensor, and these changes can be used to calculate the blood volume. The sensor is positioned downstream of the injection so that the indicator passes the sensor, with the measured diluting effect being used to determine various blood parameters. The present invention may employ the relationship between the velocity of ultrasound in blood and the constituents of the blood. That is, the velocity of ultrasound in blood is a function of, among other things, the proteins and ions contained in the blood, with the sound velocity increasing with an increase in protein concentration. Accordingly, the velocity of sound through a blood sample can be varied by diluting the blood with an indicator having different acoustical characteristics than those of the blood; for example, through the use of a saline solution that has no proteins.
The present invention provides a relatively simple technology for measuring the volume of a blood filter and creates an opportunity to control filter and dialyzer performance and give an early warning of clotting to improve the quality of a variety of procedures including hemodialysis.